Metallic glasses, unlike conventional crystalline alloys, have an amorphous or disordered atomic-scale structure that gives them unique properties. For instance, metallic glasses have a glass transition temperature (Tg) above which they soften and flow. This characteristic allows for considerable processing flexibility. Known metallic glasses have only been produced in thin ribbons, sheets, wires, or powders due to the need for rapid cooling from the liquid state to avoid crystallization. A recent development of bulk glass-forming alloys, however, has obviated this requirement, allowing for the production of metallic glass ingots greater than one centimeter in thickness. This development has permitted the use of metallic glasses in engineering applications where their unique mechanical properties, including high strength and large elastic elongation, are advantageous.
A common limitation of metallic glasses, however, is their tendency to localize deformation in narrow regions called xe2x80x9cshear bandsxe2x80x9d. This localized deformation increases the likelihood that metallic glasses will fail in an apparently brittle manner in any loading condition (such as tension) where the shear bands are unconstrained. As a result, monolithic metallic glasses typically display limited plastic flow (0.5-1.5% under uniaxial compression) at ambient or room temperature. Several efforts have been made to increase the ductility of metallic glasses by adding second phases (either as fibers or particles, or as precipitates from the matrix) to inhibit the propagation of shear bands. While these additions can provide enhanced ductility, such composite materials are more expensive to produce and have less processing flexibility than monolithic metallic glasses.
Quasi-crystalline materials have many potentially useful properties, including high hardness, good corrosion resistance, low coefficient of friction, and low adhesion. However, known aluminum-based quasi-crystals produced by solidification are too brittle to be used as bulk materials at ambient temperature. Recently, precipitation of quasi-crystalline particles was found upon annealing bulk metallic glasses Zrxe2x80x94Cuxe2x80x94Nixe2x80x94Alxe2x80x94O and Zrxe2x80x94Tixe2x80x94Cuxe2x80x94Nixe2x80x94Al. The quasi-crystalline phases in these alloys are metastable and can only be formed by annealing the amorphous precursor in a narrow temperature range between 670 K and 730 K.
In accordance with a preferred embodiment of the invention, an alloy is provided that is capable of forming a metallic glass at moderate cooling rates (less than 1000 K/s) and that also exhibits large plastic flow, namely plastic strain to failure in compression of up to 6-7% at ambient temperature. Preferably, the novel alloy has a composition of (Zr, Hf)a TabTicCudNieAlf, where the composition ranges (in atomic percent) are 45xe2x89xa6axe2x89xa670, 3xe2x89xa6bxe2x89xa67.5, 0xe2x89xa6cxe2x89xa64, 3xe2x89xa6b+cxe2x89xa610, 10xe2x89xa6dxe2x89xa630, 0xe2x89xa6exe2x89xa620, 10xe2x89xa6d+exe2x89xa635, and 5xe2x89xa6fxe2x89xa615.
In accordance with a preferred embodiment of the invention, the novel alloy may be cast into a bulk solid with disordered atomic-scale structure, i.e., a metallic glass, by a variety of techniques including copper mold die casting and planar flow casting. The as-cast amorphous solid has good ductility (greater than two percent plastic strain to failure in uniaxial compression) while retaining all of the characteristic features of known metallic glasses, including a distinct glass transition, a supercooled liquid region, and an absence of crystalline atomic order on length scales greater than two nm.
Moreover, the unique alloy may be used to form a composite structure including quasi-crystals embedded in an amorphous matrix. Such a composite quasi-crystalline structure has much higher mechanical strength than a crystalline structure.